Fluid saturation of porous materials is important to evaluate and model a reservoir in general. Saturation is the ratio of the pore volume occupied by a fluid phase to the total pore volume. Knowing initial fluid saturations will help validate simulations and confirm that assumed wettability values are reasonable. In general for the industry, current fluid characterization methods tend to be slow and error prone, particularly for very low permeability materials like shale.
In addition, characterization of the fluids, such as composition, viscosity, pour point, etc. for the oil and salinity for the brine are important factors for calibration (e.g. for resistivity) and understanding the economics and production of a reservoir.
Fluid saturation is important for understanding the economics, history, and optimization of production from a reservoir. In addition to their quantities, understanding the properties of the fluids therein helps determine the economics, guide the production methods, and determine the ultimate recovery. Brine salinity is important for calibration of petrophysical models involving resistivity data as well as preventing the use of fluids during drilling, completion, and production (e.g. drilling mud) that may cause damage to the reservoir (e.g. swelling of clays).
The standard way of determining fluid saturations is the Dean-Stark extraction technique. There are a variety of setups, but it generally involves cleaning samples in a solvent and measuring produced water. In one set-up, for example, the sample is placed in an extraction apparatus and the solvent is heated. Vapor of the solvent rises through the core and leaches out the oil and water. Water condenses and is collected, such as in a graduated cylinder. A typical solvent is toluene, miscible with the oil but not the water. Solvent and oil continuously cycle through the extraction process. The volume of water collected is recorded and when the reading becomes constant, the heating is discontinued. The water saturation (Sw) in the core is determined from direct measurements performed on the sample, such as the ratio of the measured collected water and the difference of the dry sample weight and fresh water resaturated weight, whereas the hydrocarbon saturation (So) is inferred by a calculation using the water saturation, the sample porosity, and other determined parameters for the sample. This method is time consuming, error prone (leaking, etc.) and requires a significant amount of sample. It has a higher error margin in low porosity and low permeability samples.
Determining fluid saturation via Dean-Stark limits further use of the cores for other kinds of analysis since all the native fluids have been removed from the sample before it is resaturated with fresh water.
Other methods include centrifuging out fluids. However, there is usually residual fluid left in the core that cannot be produced, such that this gives only a qualitative estimation of the fluid saturations.
Nuclear Magnetic Resonance (NMR) has been used as a non-destructive method for fluid saturations. Common methods use a two dimensional measurement that correlates the measured diffusion coefficient with the transverse relaxation time. Because gas has a higher diffusion coefficient than water, and in turn oil, the diffusion coefficient can be used to help separate system constituents. However, this method is frequently inconclusive in its evaluation of system fluids, as there are many competing effects that influence the NMR response.
Retort methods are also frequently used to determine sample saturation. This is performed by heating the sample in stages up to some high temperature. The exact times and temperatures will vary between labs. However, it is assumed that the fluids at a certain temperature arise from only a certain class of water (e.g. claybound). Retort measurements are frequently run in the span of an hour, which may not completely desiccate the samples and leads to uncertainty in the response. This technique also does not allow the further use of a sample in a given saturation state.
For conventional reservoir samples, it is frequently straightforward to obtain a water sample and determine its salinity. However, in shale reservoirs, obtaining a brine sample is often difficult. The samples do not easily produce water and the amounts tend to be small. Large quantities of sample are frequently required to estimate reservoir salinity and the extractions, via centrifugation or Dean-Stark, take extended periods of time.
Dielectric measurements are also used to estimate the sample salinity, but this requires detailed information on the sample composition and sometimes structure in order to estimate accurately. Textural effects in the sample may also lead to uncertainty in the dielectric response.
Laser induced breakdown spectroscopy (LIBS) uses a laser to ablate a tiny portion of sample. LIBS has been used to provide identification of materials and chemical compositions of solid materials. The standard for LIBS uses a q-switched solid state laser that produces a rapid pulse, typically on the order of pico- to nanoseconds in duration. Optics are used to focus the energy onto a single spot on the sample. A strong laser ablates a small portion of the sample, turning it into a high temperature plasma. The excited atoms then return to a ground state, giving off light of characteristic frequencies. The spot size vaporized by the laser can range in size from a few microns up to hundreds of microns, allowing a large range and is dependent on the optics of the system. The signal improves with larger spot size, but sacrifices resolution. While a small amount of sample is consumed, the amount is so small that it is considered to be negligible and the technique is considered non-destructive.
The wavelength of light from the plasma is typically measured in the 200 nm to 980 nm region. The resulting spectra can be analysed by multivariate data to correlate the spectra to concentration of elements. LIBS has been used previously as a method for mineralogy identification, making it an alternative to XRD and XRF methods for mineralogical analysis of samples. It has an advantage over XRF for mineralogical identification because it can measure all elements, whereas XRF is unable to detect light elements.
LIBS is able to perform depth profiling, firing the laser in the same spot, and observing the different products that are produced with increased depth. LIBS is also very rapid, taking only seconds per measurement and making it amenable for high-throughput industrial use. LIBS measurements can be rastered to produce a two dimensional map of surface composition.